1. Field of the Invention
The present invention relates to a hydraulic four-wheel driving apparatus for a vehicle.
2. Related Art
Conventionally, there is a well-known hydraulic four-wheel drive system for a vehicle (such as a lawn tractor) having a pair of left and right unsteerable (rear) wheels and a pair of left and right steerable (front) wheels, wherein the system includes a front-wheel speed-increasing mechanism for increasing drive speed of its front (steerable) wheels so as to prevent the front wheels from being dragged while the vehicle is turning because of uneven distance between the front wheels and the rear (unsteerable) wheels from the turning center of the vehicle when turning. With respect to the front-wheel speed-increasing mechanism disclosed in Japanese Patent No. 2,594,951, for example, one assembly unit of a hydraulic motor and a differential gearing is provided for driving the unsteerable rear wheels, and another for driving the steerable front wheels, wherein the hydraulic motor for the steerable front wheels is operated in association with steering operation, e.g., rotation of a steering wheel.
However, in the conventional front-wheel speed-increasing system, the hydraulic motor for driving the (steerable) front wheels is placed adjacent to the front wheels, thereby lengthening piping for connecting the hydraulic motor for driving the front wheels, to a hydraulic pump disposed in a transmission casing supporting the (unsteerable) rear wheels. Since a front axle casing for driving the front wheels is extended laterally to be vertically rotatable at its left and right ends around a center pivot, it is difficult to use a steel pipe for the piping. Accordingly, a flexible hose is used for the piping. However, the flexible hose may be expanded and contracted by hydraulic pressure flowing therein so as to cause power loss and high load imposed on the piping. Furthermore, the flexible hose, which is easily broken against an obstacle on the ground surface, is necessarily complicated in its piping so as to avoid obstacles.
Moreover, when the hydraulic motor for driving the front wheels is disposed near the front wheels, additional piping for circulating oil filled within its housing and for cooling the motor must be extended between the motor housing for the front wheels and the transmission casing (serving as an oil tank) supporting the rear wheels.
Additionally, a movable swash plate of the motor for driving the front wheels has no neutral position, that is, the relative oil-delivery direction of the hydraulic motor for the front wheels to that of the hydraulic pump is constant even if the movable swash plate is tilted in any direction. An especially complicated linkage is requested to accurately control such a movable swash plate in association with the steering operation, e.g., rotation of a steering wheel.